As a light-emitting diode (LED) that emits a visible light in a red, orange, yellow or yellowish green color, the compound semiconductor LED provided with a light-emitting layer formed of aluminum-gallium-indium phosphide ((AlXGa1-X)YIn1-YP wherein 0≦X≦1 and 0≦Y≦1) has been known heretofore. In the LED of this configuration, generally the light-emitting part provided with a light-emitting layer formed of ((AlXGa1-X)YIn1-YP (0≦X≦1, 0≦Y≦1) is optically nontransparent to the light emitted from the light-emitting layer and is formed on the substrate of a material, such as gallium arsenide (GaAs), which has no appreciably high strength.
Recently, therefore, for the purpose of obtaining a visible LED exhibiting enhanced luminance and with the object of further enhancing the mechanical strength as a device, a technique for configuring a junction-type LED by removing a nontransparent substrate material, such as GaAs, and thereafter bonding anew a support layer formed of a material transparent to the emitted light and excellent in mechanical strength more than ever has been developed. The transparent supporting layer, such as a Group III-V compound semiconductor crystal substrate, for example, is ordinarily disposed as bonded to the surface of a buffer (barrier) layer and not to the surface exposed by the removal of a nontransparent substrate, such as a layer forming a light-emitting part.
As means for effecting adhesion of a transparent supporting layer, the following methods (1) to (5) have been known heretofore.
(1) A method for directly bonding the supporting layer to a semiconductor layer while applying pressure thereto at an elevated temperature of several hundred degrees (refer to Japanese Patent No. 3230638).
(2) A method for effecting bonding by a means called wafer bonding (refer to JP-A HEI 6-302857).
(3) A method utilizing a transparent adhesive substance, such as epoxy resin (refer to JP-A 2002-246640).
(4) A method for bonding a semiconductor layer and the transparent supporting layer through a transparent electrically conductive thin film, such as of an indium-tin complex oxide (ITO) (refer to Japanese Patent No. 2588849).
(5) A method comprising the steps of mirror-polishing both a semiconductor layer and a supporting layer, bonding the two layers after removal of defiling matter and heat-treating the bonded layers (refer to JP-A 2001-57441).
The technical means of (1) which attempts to bond a transparent supporting layer directly to the surface of a semiconductor, however, necessitates elevation of temperature to a high level of 600° C. or more and application of pressure as well (“Semiconductors and Semimetals,” Vol. 48, edited and written by G. B. Stringfellow and M. George Craford (published in 1997 by Academic Press (U.S.A.)), refer to pp. 196-206). An attempt to bond a transparent supporting layer under such conditions of high temperature and high pressure leads to inducing a disadvantage in easily inflicting a crystal defect on the supporting layer because stress is exerted, for example, on a Group III-V compound semiconductor layer which possesses brittleness. When the surface of the Group III-V compound semiconductor layer to be bonded, for example, is not flat, the pressure is unevenly applied to the layer, with the result that the bonded layers will frequently form such union as is inferior in quality and deficient in strength. Further, the disadvantage of the conventional means of bonding under the conditions of high temperature and high pressure consists in the fact that an effort to bond a supporting layer which reveals difference in thermal expansion coefficient results in inducing a warp due to mechanical stress and eventually entailing occurrence of crystal defects in a large amount in the interface of union.
The bond produced by the wafer bonding means of (2), being formed with a transparent supporting layer and a Group III-V compound semiconductor layer, for example, is at a disadvantage in suffering the presence of an oxide film on the surfaces of the layers being bonded or the contamination caused by a pasting agent used for bonding to degrade strength of union and aggravate electric resistance in the interface of union. In the LED, therefore, the expected decrease of the forward voltage (Vf), for example, encounters a hindrance.
Meanwhile, the bonding means of applying or inserting a viscous adhesive material, such as epoxy resin of (3) or a transparent electrically conductive thin film of (4), between the supporting layer and the semiconductor layer being bonded, in spite of being able to lower the temperature necessary for the bonding, is at a disadvantage in suffering the intervention of a foreign material, such as the epoxy resin layer, in the interface of union to inflict the stress due to the difference in thermal expansion coefficient on the Group III-V compound semiconductor layer, and consequently induce the leak of the electric current for operating the device (device operating current) via the crystal grain boundaries generated by the stress, and prevent the junction-type compound semiconductor LED abounding in reverse voltage from being obtained with fully satisfactory stability.
Particularly, in the compound semiconductor LED, when a transparent supporting layer gaining in thickness in the direction of flow of the device operating current and additionally performing an action of diffusing the device operating current throughout the whole mass of the light-emitting layer is disposed as bonded to the LED, the difference in thermal expansion coefficient between the transparent supporting layer and the adhesive material induces separation of the transparent supporting layer from the adhesive material conspicuously. This bonding means, as compared with the means of directly bonding them together without daring use of an adhesive material, poses a problem that the transparent supporting layer cannot be bonded to the light-emitting part with fully satisfactory strength and the junction-type LED cannot be fabricated with fully satisfactory stability.
Then, the surface cleaning of (5) which resorts to mirror polishing, removal of defiling matter or the like requires a highly advanced cleaning technique and subsequently necessitates an environment of cleanliness exceptionally high enough to avoid re-contamination and renders stable fabrication difficult to achieve. Further, adjustment of the environment entails a problem of adding to the burden of cost.
This invention has been proposed in view of the foregoing state of affairs and is aimed at providing a compound semiconductor light-emitting diode capable of suppressing the occurrence of a crystal defect without exerting stress on a light-emitting part, enhancing the bonding strength between the light-emitting part and a supporting layer, further decreasing electric resistance in the interface of union and improving the forward voltage (Vf), heightening also the reverse voltage, and realizing enhancement of luminance and at providing a method for the fabrication thereof.